Mine's not wired that way. I put my switch on what looked like the grounding cable (negative to the chassis). Still had power. Hmm.....
Just for giggles, moved it to the other battery negative post. Still had power. Hmm...again.
Got another switch, one on each battery. Opened both, heard the relay click, no power.
My rig has has positive cables coming off each battery, and negatives off the inboard one, heading off into the coach. I'm not smart enough to trace them all down, but it seems each battery is wired to power different "stuff", rather than the two batteries wired in parallel. Came from the dealer this way.